Carrier-shift signal control circuits



May 27, 1947. R. s. CHAPIN 2,421,054

CARRIER-SHIFT SIGNAL CONTROL CIRCUIT Filed June 15, 1945 2 Sheets-Sheet l n May 27, 1947.

R. s. cHAPlN CARRIER-SHIFT SIGNAL CONTROL CIRCUIT Filed June 15, 1,945y

2 sheets-sheet 2 Patented May 27, l17947 Reynold Sayre Chapin, Flushing, N.- Y., assigner to Press Wireless, Inc., New York, N. Y., a corporation of Delaware Application June 15, 1945,4 Serial No. 599,719

Claims. 1 This inventionA relates to' wave signalling systems andvmore especially tol systems of the kind' wherein intelligence' signals are conveyed by carrier-shift operation.

Ther invention is-in the nature of an improvement onthe type of system disclosed inv appli-` cation of R. M'. Sprague and E. G. Fraim, Serial No. 498,278; filed August 12, 1943, now Patent No. 2,387,098, issued Oct..16, 1945.

In carrier-shift signalling systems, it is customary to. transmitdifferent signals e. g., telegraph mark and telegraph space signals, by shifting the carrier frequency a predetermined amount on either side of a normal or mean frequency. In suchr systems therefore, it is highly important that the tuning ofthe system be accurately monitoredI in order thaty the proper relationship between the two frequency shift conditions be maintained.

Accordingly, it isa principal object ofthis invention to provide an improved monitoring arrangement for carrier-shiftl signalling systems.

Another principal object is to provide an automatic alarm arrangement forcalling the atten'- tion ofV an attendant when. the tuningVA of the carrier-shiftV apparatus is abnormal.

A feature of the invention relates to a receivingarrangement for receiving carrier-shift signal transmissions and having means for operating an alarm when the tuning means is not set properly with respect to the mean or center carrier frequency.

Another feature relatesto an improved tuning control arrangement for carrier frequency shift receivers whereby a, continuous monitoring check is always available on the extent of mean carrier frequency deviation and on the amount of keyed carrier shift, and a special alarm signal is provided when theY deviationof the said mean` frequency exceeds a. predetermined percentage.

A further feature relates` to an alarm system for carrier frequency shift telegraph systems whereby the alarm. is rung when the mean carrierfrequency deviates by av predetermined percentage, and in conjunction with special circuit means to discriminate between actual signal repetition frequencies, and. meank carrier frequency deviations.

A- still further feature relates to the novel organization, arrangement andl interconnection of parts which cooperate to produce an improvedk monitoring arrangement for l.carrier frequency shift signalling systems. In the drawing,

Fig. 1 is a schematic blockdiagram or a system embodying principles-f the invention.

(Cl. 25-'0--8) l 2 Fig..2 isa detailed schematic diagram of parts of Fig. 1.

Referring to Fig. 1', there is represented by ther block l, any well-knownv arrangement forA generating a :carrier wave whosefrequency isnarranged to be shi-ftedin onedirection to represent one signal cond-ition, and in the opposite direction to representv `another signal condition. The mid-frequency between these two extremes of` shift can be referred to asthe mean frequency. Thusin the case of tevl'egraphic transmission, thel carrier` may be considered as having a mean fre-` quency F; a mark signal. can then be represented by (F-liOO) C'. P. 4S.; and ai space signal by (F-4UO`). C. PA. S. In actual use thel carrierY iS either at` its mark frequency (F4-400) or' at` its space frequency (F-400). In the absence of keying` signals,l the transmitter mayv rest either at (F4-4:80),l O'r (Fl-400)'. For a detailed description of a preferred form of carrier frequency shift generator, reference may be had to Patent No. 2,387,098, grantedfO'ctober 16, 1945. This frequency shifted carrier may b'e an audio fre"- quen'cy carrier which is shifted plus 0r" minus" the required amount with respectto the theoretical mean frequency to represent the two signalling conditions, and in the case of wire line transmissionmay be transmittedt'o a receiving station' without further' modulation. If however, the transmission is' to be effected overa radio chan-v nel; the frequency shifted audiofrequency carrier can be used as a sub-carrier to' mc'dulateva high frequency or radio carrier. It' will also',y be understoodlthat' the frequency shifted audio' f'r'e-Y quency carrier can `be used to frequency-rnodulate4k 0f 2550y C. P`. Si., andthat'for a niark' Signal the carrier is Sfl'ifte'd'to 2950 C. P. S.; and for afSpCe signal thev carrier is'` shifted toy 2150 C. P.. Sl Atthe transmitter l, the radio frequency carrier is mo dulated` with thev frequency-shifted sub-carrier in anyr Well-known manner. The frequencysl'ii'ft'ed'4 audio frequency sub-carrier is detected and amplified in-a-suitable radio receiver 2. In the case of a receiver of the conventionalsuperheterodyne type.v theY receiver is tunedY andadjusted so thatl the beat frequencyv oscillator beats withy the intermediate frequency of the receiver tc'prcd-ucea resultant audioffrequency beat equal` 2,421,054 .ifi

'I'he passed signals are then impressed upon a suitable limiter device 4, such as describedin U. S. Patent No. 2,299,937, whereby all the signals an adjusting potentiometer I8.

provided with a suitable integrating or time conthis purpose, discriminator 6 is also connected through coupling condenser I0 to the control grid II of a triode electron discharge tube I2, Whose plate electrode I3 is supplied with positive D. C. potential from the terminal I4 of the D. C. power supply, in series with the energizing Winding I5 of a suitable electromagnetic 'relay which has normally open contacts I6. The grid II is normally biased to plate current cutoff by means of a suitable negative Voltage at terminal I'I through Grid I I is also trol circuit consisting of resistor I9 and condenser are brought to substantially thefsarne "level Thereupon the signals are impressed upon a lowpass filter 5 to filter-out all components, including the noise components substantially above 2950 C. P. S. For example the filter 5 may bea 3000 C. P. S. low-pass filter. The signals from filter 5 may then be consideredas auniform amplitude tone or audio frequency carrier 052550 C. P. S.. mean frequency which shifts in frequency betWeen'ZlO C. P. S. and 2-950 C. P. Sr'

'j 'ljhesignals from lter 5 areapplied in parallel to aipairrof liscri'minator" networks 6 andA 1, which-*may be of known designv for producing re spective signals in their outputs corresponding to the marking and'spacingfrequencies. Thesefdisf oriniiriatorsr h ave substantially similar linearly sloped'section's but of opposite inclinationf Thus, discriminator 6 attenuatesjthejspace frequencyof 2150 C P. S., but exerts very little attenuation on the 2950 C. P. S signal. On'the eontrary,the discriminator "I. ,attenuatesy the mark 4frequency of 2950;'C-f P. Sibut'not the space frequency of 2150 Connecte'd to theV discriminator 6 isa peak readingvacuumftube voltmeter 8 of'zany Ywell-known type.k A similar voltmeter 9 is connected to discriminatori. The scale of each of the meters 8 and '9 .is vpreferably of the zero center ,type` and is vcalibrated in unitsplus or'minus 400 C. P, S: as indicated; Bymeans of anyWell-know'n integrating circuit,"them,eters L8 ,a; nd`9 Willj respond re' spectivelygonly to' .the larger of the twojsignals Vfromfjtheir .respective discriminatorsg Whenfthe` receiving' system is inproper tune, thati isjwhen' themean sub-carrierv frequency'is latl2550C.P 1S., both meters 18' and willjhave their pointers' restin'g fat' the center Zero'. 'Should' however the receiving system' notfb'e'properly tuned; therey isan undesirable 'shift inl the 'mean frequency, 'the extenti of this'lshif-t `Wil1 be indicated .respectively'on the rme'trs'Il 'arid 911 Itv willY beV understood.' of ourse,fthat the meters 8' and 9 havev a time constant which permits .rapid charging of thetim'e constant control 'condenser' 'at the highestlke'ying rate, and a slow discharge rate 'in comparison vto theH lowest' keying rate encountered.v Therefore,

an vattendant candetermine by an inspection Aof f Vln actual use of such a systempit is not always feasible to haveanattendant keep a continuous' visual check on ythe meters 8 and 9 to determine'A Whether? 'the tuning Vof the( receiving apparatus needs" readjustment so as to maintain itat the mean( frequency. Accordingly, -it becomes' neces' sary to provide'sonie kind of audible-'alarm-to call attention to an excessive deviation of the mean carrier 'frequency' from 'its assigned valuer- -For- Whichfis controlled by contacts I6.

2l, and likewise, winding I5 is shunted by a con- Vjoleriser-22xso ;asgto average the keying pulses to provide'more stable operation of the alarm 23 Also connectedlin series with the cathode 20 is a resistance 24 across which is shunted the signal lamp 25.

*The discriminator 1 is connected to'a'similar triode 26 having its' platefelectrode'21 connected in parallel'with electrode' I3. Tube 26 is Vcou'- pled by condenser 28 to the discriminatori and has its control grid 29 normally negatively biased to substantially'plate currentcutoifby means of the negative potential `at terminal 30and through the adjustable potentiometer -3 ILv Resistance '32 and condenser 33 are similar to resistancesl 9 4and 2| already described, anda.y -cathode-"res"i-`stor-34 is' shunted by a `signal lamp 3 5. Under vthese conditions when the receiving systemis'inftine with the mean frequencyof"2550"5C.- P. S.; neither of the tubesA I2 or 26 is 'plate conductivesufriciently to operate the `V'contactsl I6.l Therefora the control"win'f'iingSl ofthe; alarm bell 237 re eeive's insufficient c'zurrent vfrointl'ie ,sixvoltsouree at terminals 3T, 382 tooperat'ethe-alarm#V` However, `the winding 36 i-s connected inserieswith a supervisory sie r 1a 1' `1anip '3 9 whih remains lighted as long as the contacts I6 'are open. Should the. mean frequency bev 'aciidentallyrir'icreased from -Y its assigned value `of 255 0' C'.' P." S there will be---producedat this new rneanffrea quency at the ouirlitgf the .disriminawr @xga voltage of suflicient amplitude'toovercome' the negative bias! 0e grid, Il, to cause plate., .current to flow through tube l2, and l :aAi-sirig vthelighting 0f laffi'pf 1?-5-,1 .Atithe Same iimleixths platepcurrent HQWS ,through relay Windinglland, 'closescon'e' tacts I6, whereby the alarm 4bell 23 isirun closing of contacts I6 also provides" ajshunt around lamp i 39 'Which '.bCCQmSS l'gxtlgushedf Onfthey other hand, `should thejjniean frequency ac'identelly deviate ,in;@negative-directioagthere Willbe a orespending voltage applied 'from discriminator 1 tofc'ontrol grid 123 which likewise controls the alarm belle 2 3 and the ysignal light `3 5.

From the f oregoing it willbe seen that when mistuning ofv the receivingsystem'joccurs, either o f the lamps 2 5 or' '3 5 1 wi ll`jli ght depending upon which. difetionzthe ,dev'ationpfi theiriean .frequency Occurs; and bothtases `the;alarm hell 23 is` rung] The attendant can' therefore roughly retunethe 'receiver "in 'thei'correct frequency di:- rection byfr'st observing whieh'VVA ff; the lampfs 25,35', islighted.` "After prelimmymrieturi` the alarmbell 23'ceases to ringan'd bothflamp's 25 and v35 are extinguished; "The attendant'may then 'precisely tune therec'eiver 'to the 'e'igathieah frequency byobservation `vof lthe''appropriate uning meter Suor-V9.l fTh'etwo meters-Will then r' Y form typical arrangements for the discriminators 6 and 1 and typical circuits for the meters 8 and 8. The frequency-shifted sub-carrier of the above assumed mean frequency of 2550 C. P'. S. is coupledthrough transformers 58, 5|, to the respective discriminators 6 and 1, which are-oppositely poled so that one discriminator produces positive voltage when a mark signal is received, and the other discriminator produces positive voltage when a space signal is received. The inductances 52, 53, condensers 54, 55, 56, 51, and resistors 58, 59, are proportioned and adjusted to provide across the potentiometer resistance 68 a relation between discriminator input frequency and outputlvoltage such as represented by the curve A (Fig. l). Likewise, the various inductances,l condensers and resistances of discriminator 1 corresponding to those of discriminator 6, are adjusted and proportioned to provide acharacteristic curve represented by the curve B (Fig. 1).

Each of the vacuum-tube voltmeters 8 and 9 comprises a diode detector 6|, 62, to rectify the voltages applied to the control grids 63, 64; 65, 66, of the duplex triodes 61, 68, whose cathodes 69, 10; 1|, 12, are returned to ground through the respective cathode-load resistors 13, 14; 15, 16. Connected between each of the diodes 6|, 62, and their respective tubes 61, 68, are integrating circuits comprising resistors 11 and 19, and condensers 18 and 88.

The positive D. C. voltage for the plates of tubes 61, 68, is -supplied through respective balancing potentiometers 8|, 82, for balancing the dual triode elements of each 0f the tubes 61, 68. When no signals are being impressed upon either of the discriminators, the potentiometers 8| and 82 are adjusted so that the cathode voltages of cathodes 69 and 18 are equal, and the cathode voltages of cathodes and 12 are equal, thus producing no deflection of the pointers of the meters 83, 84. The meters 83 and 84 may be identical commercially available milliammeters which give for example full scale deflection on the passage of a milliampere through their moving coils. When no current passes through the meters, the meter pointers are not deflected and rest in the extreme left position represented by the dotted line so that each meter will indicate minus 400 cycles per second.

Assuming that the tubes 61 and 68 have been balanced by their respective potentiometers 8| and 82 as above described, when a signal appears at the output of the discriminators the meter pointers will be deflected to the half scale position indicating zero cycles per second on the scale. This condition obtains only so long as the receiver 2 is in the intuned condition. Additional controls in the form of potentiometers 68 and 85 may be provided so as to bring the meter pointers to the zero C, P. S. position for various amplitudes of carrier-shift deviation. Should this deviation change during actual signal transmission, both meters will change their deflection in the same direc-tion. Should the receiver 2 become detuned for any reason, one discriminator output voltage will change in amplitude in one direction and the other discriminator output voltage will change in amplitude in the opposite direction. Consequently, one of the vacuum-tube voltmeters 8, y8, will receive greater input voltage than the other. vTherefore, one of the meters will be subjected to more deflection and the other meter will be subjected to less deflection.` In other words, one meter will register an increase in frequency and the other meter will register a decrease in frequency. Thus it is possible to observe directly from the readings of the meters in which direction the retuning of the receiver tube must be effected. When the transmitter is resting on the mark or space frequency the presence of the diodes 8|, 62, permits only one of the tuning meters to be deflected. The other meter will have no deflection and will show minus 400 cycles per second on the scale. During transmission keying, the criterion for the intune condition of the receiver 2 is the existence of equal deflection on both meters 83 and 84.

It will be understood of course that the meters 83 and 84, instead of being calibrated with a center zero, can be calibrated in cycles per second from zero deflection up to maximum deflection. While the center zero calibration enables direct reading of the frequency drift in cycles per second, the alternative calibration from zero to maximum permits direct reading of the actual keyed deviation of the carrier. With this latter zero to maximum calibration, drift measurement of the mean frequency is determined by reading the frequency on the meters before drift and the frequency reading after drift.

Fig. 2 shows the signal reproducer `42 operated directly from the output of the discriminator 6 in which event the rectifying diodes 86, 81, are required. If however the circuit shown in Fig. 2 is to be used merely for tuning indication and not for signal reproduction, the detectors 86, 81, may be deleted since the necessary rectication can be effected in the diodes 6I, 62, of the A. C. vacuum-tube voltmeters 8 and 8.

While certain specific embodiments have been disclosed herein, Various changes and modiiications may be made without departing from `the spirit and scope of the invention.

What is claimed is:

1. In a signalling system of the carrier frequency shift type, means to produce signals respectively above and below a mean frequency, receiving means tunablesin the absence of intelligencesignals to the mean frequency, a pair of discriminator networks upon which said frequency-shifted signals are impressed, one of said networks selectively attenuating one of said signals, the other of said networks selectively attenuating the other signal, means to produce a monitoring voltage when the mean frequency deviates in a positive direction, means to produce another monitoring voltage when the mean frequency deviates in a negative direction, indicator means controlled respectively by said monitoring voltages, and a common alarm device which is controlled by either of said monitoring voltages when the said deviation exceeds a predetermined amount.

2. A system according to claim 1 in which the characteristic curves of said discriminators relating input frequency to output voltage have susbtantially similar linearly sloped portions but of opposite sloped sign. y

3. In a system of the carrier frequency shift type, means to produce signals respectively above and below a mean carrier frequency, receiving means tunable in the absence of signals to said mean;frequency, ,end-lmnitorine ,me-ansie deter:- minetwhenthe system Eisfdetuned; the last-mentionedmeans including anarrangement yfor, producingv la.r monitoringvoltage whichY isv vproper-- tionate .'-to- .the amount loffdetuningffa grid-controlledlelectron-:tube means normallybiasing saidetubeisubstantially to plate' current cutoff, and an alarm device-controlled-bythe plate ycurrent of-:said tube. f f f .4;.,In `-asystemfor receiving telegraph-mark andi-.space signalsl f of, the Acarrier-shift type, whereinfshifting Yfrom mark tospaceis representediby shiftingrv cf `the carrier frequency between two.iifequencyqlimits,A receiving meansv for the sniftedaf-requencycarrier,means.to ytune ,the` receiyingmeansso thatin theabsence of any signalsftj-is tuned lto a frequency midway between said ,limits.,andindicatormeans to rproduce an indication when said ltuning-:changes and the dinectionoffsuchchange. 1

f M v- 5.--In asystem for-recevingftelegraph mark and., lspace signals ofthej carrier-shift ,-type, whereimshifting from .mark tofspace is represgmtedV by,Z shifting f of Y, the carrier frequency =between two frequency limits, means to tune. 4the receivers@ that.. inthe absence rof` signals itV` is Y, f The" following' refer-encesare4v `of rrecord .in the 

